1. Field
The present invention relates to a synthetic silica glass molded body, a method of molding a synthetic silica glass molded body, and a method of inspecting a synthetic silica glass molded body, and, in more details, relates to a synthetic silica glass molded body which is used as a material for producing, for example, a photomask and an optical parts including a lens, a prism, a window, and the like which are used in an exposure apparatus using a light source such as an ultraviolet laser such as an excimer laser, an Hg lamp, and the like, and relates to a method of molding and a method of inspecting a synthetic silica molded body.
2. Description of the Related Art
In order to transfer an integrated circuit pattern such as an IC and an LSI, a downsized projection exposure apparatus (or photolithography apparatus) is mainly used. A wide exposure area and a higher resolution entirely covering such an exposure area are required for the projection optical system used in such a downsized projection exposure apparatus with an increase in the integration degree of the integrated circuit. Then, the countermeasures including the reduction in an exposure wavelength or the increase in the number of numerical aperture (NA) of the projection optical system are taken to improve the resolution of the projection optical system.
As an exposure light source, the i-line (365 nm) of an Hg lamp, as well as the KrF (248 nm) excimer laser and the ArF (193 nm) excimer laser which is a deep ultraviolet light source are mainly used at present in a liquid crystal display exposure apparatus and in a semiconductor exposure apparatus, respectively.
An optical member which can be used in such an exposure light source having a short wavelength and high irradiance is limited. In some parts of the liquid crystal display exposure apparatus provided with the above described exposure light source, a synthetic silica glass member is used as an optical member. In the semiconductor exposure apparatus provided with the above described exposure light source, a synthetic silica glass member is mainly used as an optical member. Such a synthetic silica glass member is an essential material in imaging optics of the downsized projection exposure apparatus because the synthetic silica glass member has high transmittance in an ultraviolet wavelength region and high resistance to long time exposure to the ultraviolet light.
Moreover, another important factor required to print a circuit on a wafer using a downsized projection exposure apparatus includes a reticle (photomask). In the photomask, the thermal expansion of a substrate due to the increase in the temperature is a major problem in addition to the ultraviolet transmittance and resistance, so a material having a small thermal expansion coefficient is required. For this reason, in the photomask, the synthetic silica glass member is used as the most important material.
For example, the direct method which is one of chemical vapor deposition (CVD) methods is used as a production method of such a synthetic silica glass. Here, the direct method refers to a method in which the silica glass is obtained as follows. A combustion-supporting gas (oxygen-containing gas, for example oxygen gas) and a combustible gas (hydrogen-containing gas, for example, hydrogen gas or natural gas) are mixed and burned with a burner made of silica glass. On the other hand, a high purity silicon compound (for example, silicon tetrachloride gas) as a raw gas is diluted by a carrier gas (usually oxygen gas), and the raw gas thus obtained is jetted from the center of the burner. Then, the raw gas is reacted (hydrolyzed) by the burning the oxygen gas and the hydrogen gas therearound to form fine particles of silica glass. The fine particles of silica glass are deposited on a target comprised of an opaque silica glass plate which is located beneath the burner to be turned, swung, and pulled down. At the same time, the particles of silica glass are melted and vitrified by the combustion heat of the oxygen gas and hydrogen gas. In the production method of the silica glass using the direct method as described-above, the solid is directly synthesized from a gaseous raw material. Accordingly, very high purity synthetic silica glass can be obtained. That is, the silica glass block synthesized by the CVD method has extremely high purity, high transmittance, and high resistance to ultraviolet irradiation. However, it has been relatively difficult to produce the synthetic silica glass having a desired form, especially having a large diameter, by the synthetic silica glass production method using the direct method.
On the other hand, in recent years, a synthetic silica glass member having a large surface area in a large sized lens, a photomask, a large-size flat panel display device, and the like has been necessitated. Then, in order to produce the synthetic silica glass member having a large surface area, the synthetic silica glass block obtained by the above described production method is molded by pressing it while heating. In the synthetic silica glass production method by molding and pressing the synthetic silica glass while heating, the synthetic silica glass molded body is molded as follows. The synthetic silica glass block is molded while accommodated in a mold and pressed with a pressing plate while being heated therein. Then, it is gradually cooled in the mold, and further annealed to mold a synthetic silica glass molded body having an enlarged opposed area and a predetermined form. However, in such a heating and pressing molding method, impurity contamination occurs in the synthetic silica glass due to the contact between the synthetic silica glass block and the mold which takes place in a heating and pressing molding process. As a result, there is a problem that the transmittance and irradiation resistance of the obtained synthetic silica glass molded body is reduced. Thus, it is not desirable to use the synthetic silica glass molded body in which impurity contamination has occurred as a material of an optical member for an exposure apparatus. Moreover, there is another problem that the impurity contamination is spread in a very wide region in the synthetic silica glass molded body because the impurity contamination occurs when the synthetic silica glass block is heated to a high temperature and when the temperature is maintained for a long period of time. For this reason, such an impurity contamination has been a serious problem, which causes the notable reduction in the yield of an optical member produced using the synthetic silica glass molded body as the material.
Japanese Unexamined Patent Application Publication No. 2003-81654, discloses a method of producing a synthetic silica glass which is characterized in which, in the step of heating a preform or molded body of the synthetic silica glass produced by the direct method or soot method, the heat treatment is performed while the preform or molded body of the synthetic silica glass is accommodated and heated in a container which is made of a carbon material containing Cu in a concentration of 0.1 ppm or less, and which is previously heat-treated at 1200° C. or more under a reduced pressure or an inert gas atmosphere. However, such a method of producing a synthetic silica glass is not sufficient to prevent the above described impurity contamination.
On the other hand, in some cases, the impurity contamination causes additive fluorescent light to be observed in a synthetic silica glass member at the time of exposure, as well as it causes deterioration in a quality such as transmittance and irradiation resistance. Such an observed additive fluorescent light may not necessarily be inhibited strictly as the variation in transmittance and laser damage because it does not cause the irradiance of exposure light to directly be varied unlike the variation in transmittance and laser damage. In actual, when the KrF excimer laser light and the ArF excimer laser light which have a high energy density of about 50 mJ/(cm2·pulse) or more are irradiated to the synthetic silica glass member, it actually does not happen that fluorescent light is not observed at all. This is because the sensitivity of fluorescent light is drastically high as compared to other physical properties (for example, transmittance), as well as because the structural defect such as non-bridging oxygen contained in the glass causes the fluorescent light. However, fluorescent light possibly causes flare which reduces the sharpness of a line pattern. Accordingly, it is preferable that the intensity of fluorescent light be as low as possible in practical use.
Japanese Unexamined Patent Application Publication No. 2001-114530 describes that impurities and the like which reduce the transmittance and irradiation resistance of the synthetic silica glass member in an ultraviolet wavelength area are typified by the transition metal elements including Fe, Ni, Cr, Mn, and the like, as well as, in a deep ultraviolet wavelength area such as the ArF excimer laser, alkaline metal elements such as Na and halogens such as CI also have an influence on the transmittance of the synthetic silica glass member in an ultraviolet wavelength area. As fluorescent light generated due to the impurity contamination, a fluorescent light band having a center wavelength of 380 nm generated by Na and a green color fluorescent light band having a center wavelength of 500 nm generated by Cu are typically known (See M. Shimbo et. al., Japanese Journal of Applied Physics, 1993, vol. 32, p. L671-L673 and R. Debnath and S. Kumar, J. Non-Crystal. Sol., 1990, vol. 123, p. 271-p. 274).
Japanese Unexamined Patent Application Publication No. 2001-72428 discloses a synthetic silica glass having 0.01 or less of a ratio of fluorescent light emission intensity relative to the intensity of a scattering light generated from a synthetic silica glass, the ratio being determined by irradiating the synthetic silica glass with an ultraviolet light having a wavelength ranging from 150 nm to 400 nm. Moreover, Japanese Unexamined Patent Application Publication No. Sho 60-39535 describes a silica glass screening method which is characterized in that the existence of copper in a silica glass is detected by means of fluorescent light.
Furthermore, as the fluorescent light which is emitted by a synthetic silica glass molded body, yellow (yellowish green) fluorescent light with a wide band having a center wavelength of 550 nm is observed some time after the KrF excimer laser is irradiated. However, the cause of the emission of yellow (yellowish green) fluorescent light has not been cleared as far as the present inventor investigated.